Conference warhed::astronomy

From:	VBORMC::"[email protected]" "MAIL-11 Daemon" 31-JAN-1997 08:27:32.45
To:	[email protected]
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Subj:	Planetary Astronomers Start Year With Two Discoveries

Douglas Isbell
Headquarters, Washington, DC             January 29, 1997
(Phone: 202/358-1753)
Sender: owner-press-release
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Diane Ainsworth
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)

RELEASE: 97-20

PLANETARY ASTRONOMERS START YEAR WITH TWO DISCOVERIES 

    Two newly detected members of the Solar System -- a rare 
asteroid orbiting close to Earth and a distant comet making its 
only appearance -- mark the first discoveries of the year for a 
team of astronomers at NASA's Jet Propulsion Laboratory (JPL), 
Pasadena, CA. 

    The discoveries, reported Jan. 10 by JPL planetary scientists 
Eleanor Helin, Steve Pravdo, David Rabinowitz and Ken Lawrence, 
were made possible with a few nights of clear observing weather 
and use of a sensitive, charge-coupled device (CCD) camera called 
the Near-Earth Asteroid Tracking (NEAT) system at Mt. Haleakala, 
Maui, HI. Since their initial sightings, both objects have become 
the focus of worldwide observations by astronomers in Japan, 
China, Australia, Canada, Italy and the Czech Republic.

    "This asteroid is a member of a rare class of asteroids, 
called Atens, which stay within Earth's orbit most of their 
lifetimes," said Helin, principal investigator of the NEAT 
project. "The object has a higher inclination to the plane of 
Earth's orbit than most Atens; in fact, at 31 degrees, it has the 
second highest inclination of all the Atens we've discovered." 

    The highly inclined orbit, which is unusual, may result from 
long-range interactions with the planets, or may be the outcome of 
previous orbits passing near the Earth. With the discovery of more 
Atens, the relative importance of these competing influences may 
be better understood.

    Dubbed 1997 AC11, the asteroid is a faint object with an 
absolute magnitude of 21, and probably measures about 600 feet in 
diameter.  It is only the 24th Aten to be discovered in 21 years, 
since Helin found and named the first Aten in January 1976.  With 
orbits that are smaller than Earth's, and short periods, Atens are 
in the vicinity of Earth frequently.  This closeness to Earth 
makes them more likely to impact the planet than other types of 
asteroids. 

    "Atens never wander far from the orbit of Earth and can cross 
Earth's orbit as many as four times a year," Helin said.  "1997 
AC11, for instance, has a period of 8/10ths of a year, or roughly 
9.5 months.  As we continue to observe it in coming months, we 
will be able to characterize its orbital path with more precision. 
With more precise data, we will be able to examine its potential 
for collision with Earth at some time in the future."

    Along with the newest Aten, astronomers also discovered a new 
comet, still distant but moving toward the Earth and Sun, as it 
passed through the constellation of Leo. Designated  Comet 1997 
A1, the celestial snowball is expected to make its closest 
approach to Earth on Feb. 6, passing at a distance of about 230 
million miles, but remaining visible in the night sky for several 
months thereafter.

    "This comet has traveled a long distance, originating in the 
Oort Cloud, a region far beyond Pluto's orbit which is believed to 
house trillions of incipient comets," Helin said.  "It has a 
parabolic orbit, which means it will travel through our Solar 
System once and probably never be seen again. Parabolic comets do 
not present their calling cards before arriving in the inner Solar 
System.  They appear without warning."

    At discovery, 1997 A1 was fairly dim at magnitude 19, and 
showed a weakly condensed nucleus with a diffuse halo and short 
tail, Helin said. The Minor Planet Center at the Smithsonian 
Astrophysical Observatory in Cambridge, MA, announced the 
discovery, reporting it as a parabolic comet, with an orbital 
inclination of 145 degrees from the ecliptic plane, and indicated 
that it would not pass any closer than 3.17 astronomical units 
(295 million miles) from the Sun. 

    JPL's NEAT team, in conjunction with another observing effort 
under way at the Laboratory's Table Mountain Observatory in San 
Bernardino, CA, will continue to track and characterize the comet 
over the next several months until it is no longer visible. 

    During its closest approach on Feb. 6, the newly discovered 
comet will be visible in the constellation of Cancer and brighten 
to a magnitude of about 18. Moderate-sized telescopes with CCD 
chips will be able to observe the comet, Helin said.  Astronomers 
report that the comet is continuing to outgas, or warm up and boil 
off some of its ices, as it moves toward the Sun.

    Low-resolution black-and-white images of both objects are 
posted on the Internet at the following URL:

           http://huey.jpl.nasa.gov/~spravdo/neat.html

    Discoveries of very faint or distant objects, and those 
surprisingly close by, are increasing due to the introduction of 
technologically advanced, fully autonomous CCD telescopes. The 
NEAT camera, for example, employs a very large, very sensitive 
4,096-by-4,096-pixel CCD. The camera is installed on a 39-inch 
telescope operated at the summit of Mt. Haleakala by the U.S. Air 
Force. 

    Using this powerful, fully automated system, astronomers are 
discovering many more objects than was possible in the past. The 
January observing run, for instance, produced more than 700 
asteroid sightings, including high-inclination inner-belt 
asteroids and a number of potential Mars-crossers, which will be 
confirmed after more observations become available. Total 
detections since NEAT began operations in late 1995 have climbed 
to more than 9,000 objects, of which more than 50 percent are new 
objects and more than 800 of those have received new designations.

    NEAT was built and is being managed by the Jet Propulsion 
Laboratory for NASA's Office of Space Science, Washington, DC. 

                             -end-

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From:	VBORMC::"[email protected]" "MAIL-11 Daemon" 17-FEB-1997 07:11:46.82
To:	[email protected], [email protected]
CC:	
Subj:	ESA's Hipparcos satellite revises the scale of the cosmos and . . .

I.     ESA's Hipparcos satellite revises the scale of the cosmos

The observable Universe may be about 10 per cent larger than astronomers
have supposed, according to early results from the European Space Agency's
Hipparcos mission. Investigators claim that the measuring ruler used since
1912 to gauge distances in the cosmos was wrongly marked.

This ruler relies on the brightnesses of winking stars called Cepheids, but
the distances of the nearest examples, which calibrate the ruler, could only
be estimated. Direct measurements by Hipparcos imply that the Cepheids are
more luminous and more distant than previously imagined.

The brightnesses of Cepheids seen in other galaxies are used as a guide to
their distances. All of these galaxies may now be judged to lie farther
away. At the same time the Hipparcos Cepheid scale drastically reduces the
ages of the oldest stars, to about 11 billion years. By a tentative
interpretation the Universe is perhaps 12 billion years old.

Michael Feast from the University of Cape Town, South Africa, announces his
conclusion about the Cepheids at a meeting devoted to Hipparcos at the Royal
Astronomical Society in London today (14 February 1997). It will provoke
much comment and controversy, because the scale and age of the Universe is
the touchiest issue in cosmology.

The best hope for confirming or modifying the result now rests with studies
using Hipparcos data on other kinds of variable stars. An investigation of
the variables called Miras, by Floor van Leeuwen of Royal Greenwich
Observatory, Cambridge, and his colleagues, is described at the same London
meeting. Full scientific reports on both the Cepheids and Miras have been
accepted for publication in a leading journal, the Monthly Notices of the
Royal Astronomical Society.

European teams of scientists and engineers conceived and launched the unique
Hipparcos satellite, which operated from 1989 to 1993. Hipparcos fixed
precise positions in the sky of 120,000 stars (Hipparcos Catalogue) and
logged a million more with a little less accuracy (Tycho Catalogue). Since
1993 the largest computations in the history of astronomy have reconciled
the observations, to achieve a hundredfold improvement in the accuracy of
star positions compared with previous surveys.

Slight seasonal shifts in stellar positions as the Earth orbits the Sun,
called parallaxes, give the first direct measurements of the distances of
large numbers of stars. With the overall calculations completed, the harvest
of scientific discoveries has begun. Among those delighted with the
immediate irruption into cosmology, from this spacecraft made in Europe, is
ESA's director of science, Roger Bonnet.

"When supporters of the Hipparcos project argued their case," Bonnet
recalls, "they were competing with astrophysical missions with more obvious
glamour. But they promised remarkable consequences for all branches of
astronomy. And already we see that even the teams using the Hubble Space
Telescope will benefit from a verdict from Hipparcos on the distance scale
that underpins all their reckonings of the expansion of the Universe."

The pulse-rates of the stars

Cepheid stars alternately squeeze themselves and relax, like a beating
heart. They wax and wane rhythmically in brightness, every few days or
weeks, at a rate that depends on their luminosity. Henrietta Leavitt at the
Harvard College Observatory discovered in the early years of this century
that bigger and more brilliant Cepheids vary with a longer period, according
to a strict rule. It allows astronomers to gauge relative distances simply
by taking the pulse-rates of the Cepheids and measuring their apparent
brightnesses.

Nearby Cepheids are typically 1000-2000 light-years away. They are too far
for even Hipparcos to obtain very exact distance measurements, but by taking
twenty-six examples and comparing them, Michael Feast and his colleague
Robin Catchpole of RGO Cambridge arrive at consistent statistics. These
define the relationship between the period and the luminosity, needed to
judge the distances of Cepheids. The zero point is for an imaginary Cepheid
pulsating once a day. This would be a star 300 times more luminous than the
Sun, according to the Hipparcos data. The slowest Cepheid in the sample, l
Carinae, has a period of 36 days and is equivalent to 18,000 suns.

Applied to existing data on Cepheids seen in nearby galaxies, the Hipparcos
result increases their distances. It pushes the Large Magellanic Cloud away,
from 163,000 light-years, the previously accepted value, to 179,000
light-years with the Hipparcos Cepheid corrections, an increase of 10 per
cent. Feast and Catchpole feed this result back to our own Milky Way Galaxy,
and into calculations of the age of globular clusters, which harbour some of
the oldest stars of the Universe.

The reckoning involves another kind of variable star, the RR Lyraes, and the
Hipparcos investigators arrive at an age of 11 billion years for the oldest
stars. Other estimates of the oldest stars assigned to them an age of 14.6
billion years. This seemed, absurdly, to leave them older than the Universe.
A team of astronomers using the Hubble Space Telescope recently declared the
Universe to be only 9-12 billion years old. The Hipparcos Cepheid result
increases that Hubble-inferred cosmic lifespan to 10-13 billion years.

"I hope we've cured a nonsensical contradiction that was a headache for
cosmologists," Michael Feast says. "We judge the Universe to be a little
bigger and therefore a little older, by about a billion years. The oldest
stars seem to be much younger than supposed, by about 4 billion years. If we
can settle on an age of the Universe at, say, 12 billion years then
everything will fit nicely."

Feast and Catchpole have also cleared up a mystery about the nearest and
most familiar Cepheid variable. This is Polaris, the Pole Star.
Imperceptibly to the human eye, its brightness varies at a relatively high
rate, every 3 days. That should make it, by the Cepheid rule, a feebler star
than it appears to be.

Hipparcos fixes the distance of Polaris at 430 light-years, and the
researchers conclude that Polaris pulsates with an overtone, at a rate 40
per cent faster than expected for a Cepheid of its size and luminosity.
Several other Cepheids gauged by Hipparcos also exhibit overtones. Were
these not recognized as fast pulsators they would give false impressions in
the Cepheid distance scale.

II.  The miraculous stars

Another famous variable star pulsates at more than twice the frequency that
theorists would expect. This is Mira, the prototype of the class of stars
investigated by Floor van Leeuwen and his colleagues, using the Hipparcos
data. To an unaided eye, Omicron Ceti appears and disappears in a cycle of
11 months. In the 17th Century astronomers named it Mira, the miraculous
star. Astrophysicists today interpret Mira as a senile star slightly more
massive than the Sun. It has swollen into a red giant and started
oscillating, as a prelude to greater instabilities that will in due course
fling the outer layers of the star into space.

Hipparcos fixes Mira's distance at 420 light-years. Other astronomers have
gauged the apparent width of the star, as seen from the ground, so the
Hipparcos team can compute the diameter of Mira as 650 million kilometres --
somewhat wider than the orbit of Mars. If the Sun were in Mira's state it
would swallow up the Earth and all of the inner planets.

Astronomers knew that Mira was big, but the Hipparcos result confirms that
it is too large to be oscillating in a simple fashion. Again its variation
is an overtone, and the same is true of some other variable stars of the
same type, known collectively as the Miras.

The sixteen Miras in the survey are mostly 300-1000 light-years away, at
distances more comfortably within the grasp of Hipparcos parallaxes. Before
Hipparcos, there was only one fairly good measurement of a Mira distance,
for the star R Leonis. Even in that case, Hipparcos adjusts the distance
from 390 to 330 light-years.

Patricia Whitelock of the South African Astronomical Observatory played a
prominent part in the Mira study. In preparation for the Hipparcos data,
observations of selected Miras from South Africa and Russia, with infrared
instruments, assessed the extent to which they are dimmed by dust. Taking
this effect into account, as well as the occurrence of overtones, the team
arrives at a cosmic distance scale. As with the Cepheids, they can deduce
distances by comparing the brightness of a Mira with its period of
variation.

Applied to the Large Magellanic Cloud, where Miras have been detected, the
Hipparcos Mira scale puts the galaxy at 166,000 or 171,000 light-years,
depending on the method of calculation preferred. This result is
intermediate between the commonly accepted distance to the Large Magellanic
Cloud and the new result from the Hipparcos Cepheid scale.

"Frankly the Cepheids are at the limit of the useful range of Hipparcos, for
distance measurements," comments Floor van Leeuwen. "And as for the Miras,
ours is the very first attempt to gauge the absolute distance to another
galaxy via parallax measurements on this type of star. So I think we should
be grateful to Hipparcos, that our earliest answers are in the right
ballpark and in fairly good agreement, without being hasty in drawing
cosmological conclusions."

III.  Only the beginning

Michael Perryman, ESA's project scientist for Hipparcos, anticipates a warm
debate among astronomers. Should the Hipparcos Cepheid results be taken at
face value, with all their implications for the size and age of the
Universe? He remains confident that the issue will be settled by other
results quarried from the Hipparcos data.

Further Hipparcos studies of variable stars, including the RR Lyraes, are in
progress. Also relevant to the distance scale are differing quantities of
heavy elements present in stars of different ages, which can affect their
luminosities. Any remaining confusion on this point will be dispelled by
mainstream Hipparcos research devoted to the basic astrophysics of stars of
different ages of origin, and at different stages of their life cycles.

"Until Hipparcos, the cosmic distance scale rested on well-informed
guesses," Michael Perryman says. "The distances we now have, for stars of
many kinds, provide for the very first time a firm foundation from which to
gauge the distances of galaxies. The work has only just begun. If it should
turn out that the Cepheids have given the final answer straight away, that
might be surprising. But there will be no reason for astonishment when
Hipparcos's direct measurements of stellar distances lead to a revised scale
for the Universe."

The Hipparcos Cepheid scale is due to be debated in London today and in
Seattle on 17 February, when Michael Feast will speak at the annual meeting
the American Association for the Advancement of Science. It will also be one
of the hot topics at ESA's Hipparcos Symposium in Venice,13-16 May.

The Venice meeting will celebrate the release of the Hipparcos and Tycho
Catalogues to the world-wide astronomical community. It will also offer the
first overview of results obtained by the groups who have had early access
to the data, by virtue of their contributions to the Hipparcos mission. The
subjects range from the Solar System and the Sun's neighbours among the
stars, through special stars and the shape and behaviour of the Milky Way
Galaxy, to the link between the starry sky of Hipparcos and the wide
Universe of galaxies and quasars.

Further notifications about the Venice Symposium will be distributed to the
press in due course. Meanwhile information about Hipparcos is accessible on
the World Wide Web: http://astro.estec.esa.nl/Hipparcos/hipparcos.html

Contact phone numbers:

Simon Vermeer, ESA PR (Science), Paris, +33 1 53 69 71 06 Prof. Michael
Perryman, Hipparcos project, +31 71 565 3615



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From:	VBORMC::"[email protected]" "MAIL-11 Daemon" 15-MAR-1997 20:54:14.45
To:	[email protected]
CC:	
Subj:	Neptune May Be Bubbling Up Ethane, Not Making Diamonds

University of Arizona News Services

----------------------------------------------------------------------------
From: Lori Stiles, UA News Services, 520-621-1877, [email protected]
Contact(s): William B. Hubbard, 520-621-6942, [email protected]
----------------------------------------------------------------------------

March 6, 1997

Neptune may be bubbling up ethane, not making diamonds

Neptune may be bubbling ethane gas into its atmosphere rather than producing
layers of diamonds at its core, a group of Italian physicists now suggest.

But we won't really know what's going on until we send a probe to the giant
blue planet, possibly in the first decade of the coming millennium,
according to planetary sciences Professor William B. Hubbard of The
University of Arizona in Tucson. Hubbard, who chairs one of the "campaign
strategy working groups" that advise NASA on the formation of the space
agency's future science mission strategy, wrote a perspective on "Neptune's
Deep Chemistry," published Feb. 4 in Science.

Hubbard's group is named the "Astrophysical Analogues in the Solar System"
group -- a name which reflects the fact that what scientists can learn about
our own giant planets will help them better understand giant planets being
found in other solar systems.

One of the big questions in planetary science about Neptune is, what happens
to all the methane in the planet's atmosphere? Neptune has much more methane
in its atmosphere than do the two, much larger planets, Jupiter and Saturn.
Methane is what gives Neptune its striking blue color.

Below Neptune's atmosphere is a vast, dense-liquid ocean of water, ammonia
and methane -- a region called "ices," ironically, because the ocean is at
least half the temperature of the sun. Primordial heat left over from
planetary formation is why a planet about 2.8 billion miles from the sun has
an ocean that is so hot.

Until Italian physicists published results of their computer simulations
(Science, Feb. 4), a leading theory had been that when methane reaches this
"ices" region, high temperatures and pressures produce a chemical reaction
that releases hydrogen to the atmosphere and crystals of pure carbon --
diamonds -- that sink as sediment toward the center of the planet.

The Italians' results suggest what more plausibly happens is that methane is
converted into the colorless, odorless, flammable hydrocarbon gas called
ethane, which bubbles up from the ocean and into Neptune's atmosphere.
Observers do see ethane in Neptune's atmosphere. It previously was
interpreted as a result of the action of ultraviolet sunlight on the
planet's atmospheric methane.

Neptune still may be forming diamonds according to the scenario earlier
described, Hubbard said. But if it's happening, it's happening at much
deeper layers within the planet.

Can De Beers rest easy? Only a probe will tell.



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From:	VBORMC::"[email protected]" "MAIL-11 Daemon" 27-MAY-1997 05:37:36.77
To:	[email protected]
CC:	
Subj:	[ASTRO] Discovery of New Galaxy in the Local Group: The Antlia Dwarf Galaxy

Discovery of New Galaxy in the Local Group: The Antlia Dwarf Galaxy

Astronomers in Cambridge have discovered a new near-by galaxy belonging to
the Local Group of Galaxies. It was found in the little-know southern
constellation of Antlia (the Air Pump) and so has been named the 'Antlia
Galaxy' by its discoverers. Astronomers had previously overlooked the Antlia
Galaxy because it appears so dim relative to the background of the night
sky. It is in a region of space previously thought to be devoid of nearby
galaxies and is important for understanding our Local Group because there
are very few members that are isolated in this way. Most of the smaller
galaxies in the Local Group are satellites of our own Milky Way and the
Andromeda Galaxy, which are much larger spiral galaxies. Being on its own,
the Antlia Galaxy has not been disturbed and distorted by the gravity of a
much more massive neighbour. It will help astronomers understand more about
the nature of undisturbed dwarf galaxies, and how the Local Group formed and
has developed over cosmic history.

In the course of the same study, the researchers also found a second
previously unknown dwarf galaxy, lying just beyond the Local Group. They
have called this one the 'Argo' Galaxy. It lies in the constellation Carina
(the Keel). Since there is already a Carina Galaxy, to avoid confusion, the
discoverers invoked the name of the obsolete constellation Argo Navis (the
Argonauts' ship), which was divided up into several smaller constellations
including Carina.

The discoveries were made by two research students, Alan Whiting and George
Hau, working at the Institute of Astronomy, University of Cambridge, with Dr
Mike Irwin of the Royal Greenwich Observatory, and were made public at the
National Astronomy Meeting in Southampton on 10th April 1997.

Dr Irwin commented, 'Most galaxy surveys have been biased towards finding
galaxies with high surface brightness. Extreme dwarf and low surface
brightness galaxies are difficult to find and are generally found only
locally (astronomically speaking). Most of the extreme examples are in the
Local Group. But despite their unassuming appearance, dwarf galaxies hold
the key to many questions about the formation, structure and evolution of
galaxies. Dwarf galaxies also tell us important facts about the distribution
and nature of dark matter, and about star formation in regions of space
where the distribution of material is not very dense.'

The researchers had started by making a visual inspection of 894 large
survey photographs of the southern sky taken by the UK Schmidt Telescope in
Australia. They searched the photographs for large, dim, diffuse objects
that no-one had catalogued before, which could possibly be overlooked
galaxies. Using an instrument at the Royal Greenwich Observatory (the PDS
microdensitometer), they digitised the images of the candidate galaxies and
studied them more closely to confirm their potentially interesting nature.
Then in March 1997, they took their final list to the Cerro Tololo
Inter-American Observatory in Chile and obtained CCD images on the 1.5-meter
(60-inch) telescope. The results showed two of the objects clearly resolved
into stars.

>From the red giant stars visible in the Antlia Galaxy, Whiting, Hau and
Irwin realised they had identified a new member of the Local Group. It seems
to be similar to other dwarf spheroidal galaxies. The brightest stars are
easily resolved and the galaxy appears quite smooth and devoid of any
obvious concentrations of stars or possible clusters. There are no obvious
star-forming regions and no young hot blue stars apparent in any of the deep
CCD images taken. The range of colours and magnitudes of the stars in the
Antlia Galaxy is typical of other known dwarf spheroidal galaxies which are
satellites of the Milky Way.

Comparison of the new discovery with these and other nearby galaxies led the
team to conclude that the distance of Antlia is close to 3 million light
years (1 megaparsec). It is most similar in appearance to the Tucana dwarf
galaxy, the only other isolated dwarf spheroidal galaxy known in the Local
Group. The apparent size of the Antlia Galaxy on the deep CCD images
suggests that its diameter is only 4000 - 6000 light years (1-2
kiloparsecs). It probably only contains a million or so stars placing it
firmly at the faint end of brightness range covered by galaxies. (By
comparison, the Milky Way Galaxy is about 100,000 light years across and
contains some two hundred billion stars.)

The Antlia dwarf is a very intriguing object. It is similar to the extreme
dwarf spheroidals orbiting the Milky Way and Andromeda, and yet is
relatively isolated in the Local Group. Being far away from the large Local
Group galaxies, Antlia will help to put limits on the age and total mass of
the Local Group by means of the 'timing argument'. According to Big-Bang
cosmologies, all the members of the Local Group were born close together in
space but moving relative to each other. Knowing their present distances and
velocities, and the masses of the larger member galaxies, makes it possible
to work backwards and decide how old the universe. This method does not
involve knowing the Hubble parameter, H0, which relates to the expansion of
the universe as a whole.

----------------------------------------------------------------------------

Note: The Local Group of Galaxies

The Local Group was first recognised by Edwin Hubble in the early days of
extragalactic research when distances to galaxies were first being measured.
There was a distinct difference between those galaxies that resolved easily
into stars and those that did not, implying that the Milky Way Galaxy is
part of a small local cluster of galaxies that condensed out of the general
expansion of the Universe. We now recognise approximately 30 members of this
condensation, which we refer to as the Local Group. The Local Group is
dominated by two large spiral galaxies, the Milky Way and the Andromeda
Galaxy, which between them contain most of the group's mass and give out
most of the light. Nearly all the known smaller members of the Local Group
are satellite galaxies orbiting these two.

The members of the Local Group discovered most recently are: Tucana -- an
unusual isolated dwarf galaxy discovered in 1990 by Lavery on the outer
fringes of the Local Group; Sextans -- the 8th dwarf satellite galaxy to be
found orbiting the Milky Way, discovered in 1990 by Irwin; Sagittarius dwarf --
the 9th dwarf satellite galaxy of the Milky Way, caught in the act of
being tidally destroyed and incorporated into the Milky Way, discovered by
Ibata, Irwin and Gilmore in 1994.

----------------------------------------------------------------------------

Contact

     Dr Mike Irwin (Royal Greenwich Observatory, Cambridge UK)
     Phone: (0)1223 337524, e-mail: [email protected]

----------------------------------------------------------------------------

Images

Colour images of the Antlia Galaxy and the Argo Galaxy will be available at
the following WWW site (expected to be in place no later than Wednesday
evening, 9th April)

   * http://www.ast.cam.ac.uk/~mike

Limited numbers of hard copies (as dye sublimation prints) are available on
request from the RAS PRO (Jacqueline Mitton).

Captions to the images...

     NEW GALAXY IN THE LOCAL GROUP
     The Antlia dwarf galaxy, a member of the Local Group of galaxies,
     discovered in 1997. This image was made by combining three separate
     images made through colour filters with a CCD camera to give an effect
     close to real colour. The telescope used was the 1.5-metre at the Cerro
     Tololo Inter-American Observatory in Chile. The Galaxy is about 3
     million light years away, around 5000 light years across, and contains
     roughly a million stars. Individual bright stars are clearly resolved.

     Credit: Mike Irwin (Royal Greenwich Observatory), Alan Whiting and
     George Hau (University of Cambridge).

     NEWLY DISCOVERED 'ARGO' DWARF GALAXY
     The Argo dwarf galaxy, located just beyond the Local Group at a
     distance of about 13 million light years, discovered in 1997. This
     image was made by combining three separate images made through colour
     filters with a CCD camera to give an effect close to real colour. The
     telescope used was the 1.5-metre at the Cerro Tololo Inter-American
     Observatory in Chile. Argo is a typical irregular dwarf galaxy. Hot
     blue star-forming regions and red supergiant stars are visible in the
     image.

     Credit: Mike Irwin (Royal Greenwich Observatory), Alan Whiting and
     George Hau (University of Cambridge).



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From:	VBORMC::"[email protected]" "MAIL-11 Daemon" 27-MAY-1997 05:26:54.90
To:	[email protected]
CC:	
Subj:	[ASTRO] Unexpected Cloud Of Antimatter Above The Galactic Center

University Relations
Northwestern University

4/24/97

CONTACT: Chris Chandler at (847) 491-3115 or by e-mail at [email protected]

FOR RELEASE: Immediate

Unexpected Cloud Of Antimatter Above The Galactic Center

EVANSTON, Ill. -- New maps of gamma rays from NASA's Compton
Gamma Ray Observatory show evidence of a previously unknown and
unexpected cloud of positrons, a form of antimatter, extending
some 3,000 light years above the center of our Galaxy.

The researchers expected the maps to show a large cloud of
antimatter near the galactic center and along the plane of the
Galaxy, caused by the explosions of young massive stars. The maps
show that gamma ray activity, but, surprisingly, they also show a
second cloud of antimatter well off the galactic plane.

"The origin of this new and unexpected source of antimatter is a
mystery," said William R. Purcell, research scientist and
assistant professor of physics and astronomy at Northwestern
University.

"The antimatter cloud could have been formed by multiple star
bursts occurring in the central region of the Galaxy, jets of
material from a black hole near the Galactic center, the merger of
two neutron stars, or it could have been produced by an entirely
different source," said James D. Kurfess, head of the Gamma and
Cosmic Ray Astrophysics Branch at the Naval Research Laboratory.

The researchers presented their findings on Monday, April 28, at
the 4th Compton Symposium being held in Williamsburg, Va. The
results have been submitted for publication in the Astrophysical
Journal. A second paper presented at the conference, entitled "The
Annihilation Fountain in the Galactic Center Region," examines
theoretical models for one possible source of the antimatter --
star bursts in the central region.

The maps of the positrons were produced by NASA's Compton Gamma
Ray Observatory, which was launched into orbit in April of 1991.
One of the instruments on board, called the Oriented Scintillation
Spectrometer Experiment (OSSE), is sensitive to gamma rays
produced by the annihilation of positrons, the antimatter
counterpart of the ordinary electron.

These gamma rays have an energy of 511,000 electron volts, or
about 250,000 times the energy of normal visible light. They are
produced when positrons (antimatter) and electrons (matter)
collide and annihilate, converting all of their mass into energy
according to Einstein's famous equation E = MC 2 .

The center of our Galaxy is located about 25,000 light years away
in the direction of the constellation Sagittarius. Because of the
intervening interstellar dust and gas, the center cannot be
observed in normal visible light. The dust and gas, however, are
transparent to gamma rays. Since the Earth's atmosphere absorbs
gamma rays, the gamma ray telescopes must be carried by high
altitude balloons or on spacecraft such as GRO.

"By combining all our observations from the direction of the
center of our Galaxy, we have been able to generate maps of where
annihilation is occurring," Purcell said. "Variations in the
number of 511,000 electron volt gamma rays would suggest the
presence of a single source of positrons, such as a massive black
hole," he added. "Unfortunately, we have not yet seen any such
variations, but what we do see is perhaps more exciting -- an
unexpected cloud of positrons."

"It's possible that this mapping effort could turn up evidence for
other unexpected clouds of positrons," Kurfess said. "We will keep
monitoring the center of the Galaxy in the hope of seeing evidence
for a black hole 'turning on' and producing positrons," he added.

Positrons, and antimatter in general, are thought to be relatively
rare in the Universe. There are several ways in which positrons
can be created, however. One way is through the decay of naturally
occurring radioactive elements. Such radioactive materials can be
created in astrophysical sources such as supernovae, novae, and
Wolf-Rayet stars, which are massive stars having violent surface
activity.

Because these objects are relatively common in the Galaxy, the
radioactive materials, and so the resulting positrons, will be
distributed throughout the Galaxy. It's very likely that the same
types of stars responsible for creating these radioactive
materials were also responsible for creating all of the matter
making up the Earth.

Another way positrons might be created is when matter falls into
the extremely high gravitational field of a black hole. As matter
falls into the black hole, its temperature will increase until it
is hot enough to create pairs of positrons and electrons which
then may stream away from the black hole at high velocities. The
number of positrons created near a black hole may change abruptly
as blobs of matter fall into the black hole from a companion star,
while the number of positrons created by radioactive decay would
be steady over long periods of time.

A third possibility is that within the past million years this
region was the site of a merger of two neutron stars which created
a massive galactic fireball. Such events are widely believed to
cause the enigmatic gamma-ray burst phenomenon which has baffled
astronomers for over 20 years.

Because the Universe appears to contain more matter than
antimatter, however, once the positrons are created it is only a
matter of time before they are destroyed. The positron is the
anti-particle of the electron, so when a positron and electron
collide they annihilate, converting the entire mass of the
positron and electron into energy in the form of gamma rays. In
many cases the resulting gamma rays have an energy of 511,000
electron volts, so the detection of these gamma rays indicates the
presence of annihilating positrons.

Gamma rays having an energy of 511,000 electron volts were first
observed from the direction of the center of our Galaxy in the
early 1970s. Then, in the early 1980s, observations seemed to show
a sharp decrease in the number of 511,000 electron volt gamma rays
emanating from the direction of the center of our Galaxy. Such a
sharp decline in the observed number of 511,000 electron volt
gamma rays indicates the annihilating positrons are being produced
by a small, discrete source.

One possibility that generated intense interest in the scientific
community is that the rays originate in the vicinity of a black
hole, dubbed the "Great Annihilator," located at or near the
center of the Galaxy.

The launch of the GRO spacecraft began a new era in our
understanding of the source of positrons in our Galaxy. The OSSE
instrument, 10 times more sensitive than earlier gamma ray
experiments, is providing scientists with the first opportunity to
undertake comprehensive observations of the distribution and
variability of the sources producing the positrons in the Galaxy.
To date, OSSE has spent over one year mapping the distribution of
the 511,000 electron volt gamma rays coming from the center of our
Galaxy and searching for variations in the number of gamma rays
observed.

The OSSE experiment team is headed by Kurfess and includes Purcell
and a team of researchers at Northwestern University headed by
Melville P. Ulmer, professor of physics and astronomy. Other
scientists involved in the 511 keV mapping study included Robert
Kinzer and Jeffrey Skibo from the Naval Research Laboratory; David
Dixon from the University of California, Riverside, Institute for
Geophysics and Planetary Physics; Lingxiang Cheng and Dr. Marvin
Leventhal from the University of Maryland Astronomy Department;
David Smith from the University of California, Berkeley, Space
Sciences Laboratory; Jack Tueller from the NASA Goddard Space
Flight Center; and Michael Saunders from the Stanford University
Systems Optimization Laboratory.

For More Information, Contact:

William R. Purcell Department of Physics and Astronomy
Northwestern University
Phone: (847)491-7851
FAX: (847)491-3135
e-mail: [email protected]

James D. Kurfess Naval Research Laboratory
Phone: (202)767-3182
FAX: (202)767-3182
e-mail: [email protected]

More information is available at
http://www.astro.nwu.edu/astro/purcell/511kev_press_release

University Relations
Northwestern University

4/24/97

CONTACT: Chris Chandler at (847) 491-3115 or by e-mail at [email protected]

FOR RELEASE: Immediate



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From:	VBORMC::"[email protected]" "MAIL-11 Daemon" 27-MAY-1997 05:13:46.21
To:	[email protected]
CC:	
Subj:	[ASTRO] The "Annihilation Fountain" In The Galactic Center Region:	         Interpreting the Cloud

April 28, 1997

        The Annihilation Fountain In The Galactic Center Region:
                     Interpreting the Cloud

(Washington, DC -- April 28, 1997) -- An unexpected cloud of antimatter
annihilation radiation was discovered by a team led by Naval Research
Laboratory (NRL) and Northwestern University researchers using data obtained
with the Oriented Scintillation Spectrometer Experiment (OSSE) on the
Compton Gamma Ray Observatory. This discovery points to the existence of a
hot fountain of gas filled with antimatter electrons rising from a region
that surrounds the center of our galaxy. The nature of the furious activity
producing the hot antimatter-filled fountain is unclear, but could be
related to prolific star formation taking place near the large black hole at
our galaxy's center. Other possibilities include winds from overweight stars
or black hole antimatter factories.

The interpretation of this major discovery is presented at the Fourth
Compton Symposium in Williamsburg, Virginia on Monday, April 28th by
Drs. Charles Dermer and Jeffrey Skibo, both of NRL. They note that the
gamma-ray observations permit us to see clearly, for the first time, a new
part of our galaxy made of a hot column of gas filled with antimatter
electrons, and they argue that the antimatter electrons come from newly
created elements produced by exploding stars formed near the center of our
galaxy. "It is like finding a new room in the house we have lived in since
childhood," comments Dr. Dermer. "And the room is not empty -- it has some
engine or boiler making hot gas filled with annihilating antimatter. No one
is certain whether the antimatter comes from exploding stars, black holes or
something entirely different, and that is what makes this discovery so
exciting."

Background

We live on the outskirts of an undistinguished spiral galaxy called the
Milky Way. Our Solar System lies over 25,000 light years from our galaxy's
center, which is blocked from the view of optical telescopes by intervening
gas and dust. Yet we can still look at the inner parts of our galaxy by
peering with telescopes sensitive at radio, infrared, X-ray and gamma-ray
wavelengths.

Evidence points to the existence of a black hole with the mass of a million
Suns at the very center of our galaxy. Strangely enough, unlike in other
galaxies which harbor huge black holes, very little light comes from this
source. Some 300 light years from the galactic center lies another black
hole called the Great Annihilator which, though weighing in at only 10-100
times the mass of the Sun, produces X-rays and jets seen by their radio
emissions. The outflowing jets could be made of antimatter. Huge dense
clouds of gas also surround the galactic center. Prolific star formation,
powerful stellar winds from massive stars, and supernovae are all found here.

In 1991, NASA launched the Compton Gamma Ray Observatory. One of its goals
was to view our galaxy in the light of gamma rays. Gamma rays are extremely
energetic light photons produced by high-energy particles, by the decay of
excited nuclei, and also produced when matter annihilates with antimatter.
Antimatter cannot be found in large quantities on Earth because it would
instantly vaporize anything it came into contact with. All evidence points
to our universe being composed almost entirely of normal matter, though
opinions differ on this. It was therefore unexpected to find a cloud of
annihilating antimatter above the center of our galaxy.

Using OSSE on the Compton Observatory, developed by a team led by Dr.
James Kurfess of NRL, antimatter positrons were found to be annihilating with
normal matter electrons at an astonishing rate. Scientists are speculating
on the origin of this antimatter, with a ``black-hole lobby" favoring
antimatter production in the jets of black holes.

Other scientists favor freshly synthesized radioactive material in stellar
explosions being spewn up above our galaxy in an annihilating fountain of
gas. Drs. Dermer and Skibo favor the latter scenario, because exploding
stars will eject large quantities of hot gas made up of normal matter. This
hot gas provides a target with which the antimatter electrons can
annihilate.

Whatever the true situation, a hot gas appears to be heated and blown out
from the region near the center of our galaxy. This antimatter-filled gas
traces a new feature of our Milky Way, essentially unknown until now.
Activity hidden behind vast clouds of dust and gas can be seen in the light
of its gamma radiation, giving us a new view of our home galaxy.

                                    -30-

A full set of figures may be found here.

Fountain Simulation caption: Theoretical model of the fountain of
annihilating antimatter electrons. The broad horizontal emissision is
annihilation radiation from the disk of the galaxy. The bright circular
region is annihilation radiation from the galactic center. The newly
discovered fountain of annihilation radiation points upward, away from the
plane of the galaxy.



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From:	VBORMC::"[email protected]" "MAIL-11 Daemon" 27-MAY-1997 05:13:27.80
To:	[email protected]
CC:	
Subj:	[ASTRO] Comet Crash: Teraflops Computer Simulates Colossal Comet Impact	Into Ocean

Comet Crash: Teraflops Computer Simulates Colossal Comet Impact
Into Ocean

Sandia exercise a tuneup for world's fastest computer

By Ken Frazier
Sandia Lab News Editor

ALBUQUERQUE, N.M. -- Even before it's at full strength, the new
teraflops (trillion operations per second) supercomputer at Sandia
National Laboratories is making a big splash worldwide.

During the initial testing of the new computer, Gil Weigand, U.S.
Department of Energy (DOE) Deputy Assistant Secretary for
Strategic Computing and Simulation, requested that Sandia complete
a simulation that would be of general interest to the scientific
community. For this reason, and also to generate unclassified data
to test innovative visualization techniques, Sandia scientist
David Crawford has carried out a computational simulation of a
major cosmic event of potential significance to all people on
Earth: What would happen if a kilometer-wide comet struck the
ocean?

A kilometer is about the size of the largest fragment of Comet
Shoemaker-Levy 9 that crashed into Jupiter in 1994 -- an event
that was also the subject of highly praised computational
simulations by Crawford and colleague Mark Boslough. The close
correspondence between those predictions of a visible plume rising
above the rim of Jupiter and the actual plume as observed by
astronomers lent even more confidence to the accuracy of the
Sandia simulation codes.

The new calculation again used Sandia's CTH "bang and splat" shock
physics code, but this time the simulation was run on 1,500
processors of the new Intel Teraflops computer being installed at
the Labs. That's only one-sixth of the expected final 9,000-processor
configuration.

The calculation assumed a 1-kilometer-diameter comet (weighing
about a billion tons) traveling 60 kilometers per second and
impacting Earth's atmosphere at about a 45 angle. This is small
as far as comets go (the massive Comet Hale-Bopp weighs about ten
trillion tons).

The problem was divided into 54 million zones and ran for 48
hours.

The results, although dramatic, pretty much confirm earlier
predictions about a comet impact, but they do so with much finer
resolution in three dimensions than has ever before been possible.

A revolution in science

"What's unique about this is we can now do three-dimensional
simulations on the Intel teraflops computer that can fully resolve
all the physics of the impact," Crawford says.

The fully-resolved three-dimensional resolution is extraordinary.

"It's like an astronomer getting a new, more powerful telescope,"
says Boslough. "I think it's a major step forward in science." He
said the capability raises computational simulations to the status
of a third branch of scientific inquiry equal to, and complementary to,
experimentation and theorizing.

"It really is a revolution in science," Crawford says. "A lot of
major breakthroughs in science are going to come from these kinds
of supercomputers." He notes that the comet-impact simulation is
something that can't be done any other way. "It's almost like
doing an experiment -- one you could never do. One you would never
want to do."

300-gigaton impact

Here's what the new Sandia simulations show.

The simulation starts with the comet 30 kilometers above the
surface. The comet produces a strong luminescent bow shock in the
atmosphere as it speeds downwards. Seven-tenths of a second later
it hits the ocean with an impact energy of 300 gigatons of TNT --
about 10 times the explosive power of all the nuclear weapons in
existence in the 1960s at the height of the Cold War -- forming a
large transient cavity in the ocean and a dent in the ocean floor.
The comet itself is almost instantaneously vaporized, along with
300 to 500 cubic kilometers of ocean. This high-pressure steam
explosion rises into the atmosphere. Comet vapor and water vapor
are ejected into ballistic trajectories that will take it around
the globe, with some of it even achieving escape velocity.

Low-lying areas like Florida would indeed be washed over, but
Crawford says the event is very close to the size threshold at
which impact experts expect that a global catastrophe could occur,
by screening out much sunlight for long periods of time and
disrupting agriculture, among other effects. "Simulations of this
kind can help pin down that energy threshold and help answer the
question: Is it a regional or global catastrophe?"

Low-probability, high consequence

What is the likelihood of something like this happening?
Boslough says the estimated probability is that an asteroid or
comet with this energy strikes Earth about once every 300,000
years. Another way of looking at it is that there is about a 1 in
3,000 chance of its happening in a given century. "It's a
low-probability, high-consequence event," he says. "But if it did
hit, the probability of your becoming a victim would be high."

Sandia's teraflops computer is a joint development of DOE, Sandia,
and Intel. It represents the initial goal of DOE's Accelerated
Strategic Computing Initiative (ASCI), a ten-year program designed
to move nuclear weapons design and maintenance from a test-based
to simulation-based approach. DOE announced last December, when
the machine was then still at Intel, that the
one-trillion-operations-per-second breakthrough had been achieved.

The full machine is expected to have a peak performance capability
of 1.8 teraflops, or 1.8 trillion mathematical operations per
second. DOE and the weapons labs are developing continually more
powerful supercomputers to simulate the complex 3-D physics
involved in nuclear-weapon performance and to accurately predict
the degradation of nuclear weapon components as they age in the
stockpile.

The comet-simulation was essentially a test of the teraflops
machine's capabilities. "This is an exercise for the computer,"
says Boslough (who notes that he's also using it for a
weapon-component simulation), "but we wanted to do something that
people would be interested in."

Sandia is a multiprogram DOE laboratory, operated by a subsidiary
of Lockheed Martin Corp. With main facilities in Albuquerque,
N.M., and Livermore, Calif., Sandia has major research and
development responsibilities in national security, energy, and
environmental technologies and economic competitiveness.

--30--

Color images and animation are available at this Web site:
http://www.sandia.gov/1431/COMETw.html



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From:	VBORMC::"[email protected]" "MAIL-11 Daemon" 27-MAY-1997 05:09:42.05
To:	[email protected]
CC:	
Subj:	[ASTRO] Violence in the Very-High-Energy Gamma Ray Universe

April 28, 1997

Violence in the Very-High-Energy Gamma Ray Universe

Editors: News releases announcing the Whipple Observatory Gamma Ray
Collaboration's discoveries are being issued simultaneously by the
Harvard-Smithsonian Center for Astrophysics, Iowa State University and
Purdue University.

Mount Hopkins, Arizona -- The sky seen by very-high-energy gamma-ray
astronomers turns out to be a far different place than seen by astronomers
at other wavelengths. In this universe, extraordinarily violent distant
galaxies are the brightest objects rather than the Sun, planets or stars.
Although detectable in the other more familiar wavebands of astronomy
(radio, infrared, visible, ultraviolet, x-ray) these galaxies emit the bulk
of their energy in the gamma-ray bands. Not surprisingly, such violent
emission is unsteady with the observed gamma-ray signal varying on
time-scales of months, days and even hours.

Although the orbiting Compton Gamma Ray Observatory has been remarkably
successful in sensing the universe at wavelengths beyond x-rays, even the
powerful instruments on board this mission cannot detect the highest
gamma-ray photon energies. Only ground-based gamma-ray telescopes can
measure these very-high-energy gamma rays, although indirectly, which reveal
a population of distant galaxies displaying unprecedented violence.

In February, 1997, an international collaboration of astronomers working at
the Smithsonian's Whipple Observatory in Arizona, saw the gamma-ray signal
from one such galaxy, Markarian 501, increase by a factor of ten. European
gamma-ray astronomers working in the French Pyrenees and on the Spanish
island of La Palma quickly confirmed the high flux level. Although the flux
varies greatly, the average level continues to be high. Thus, for the past
two months, the previously unremarkable object, Markarian 501, has been the
brightest object in the high-energy gamma-ray sky. It dwarfs the emission
from nearby supernova remnants such as the Crab Nebula in our own galaxy.

Markarian 501 is a giant elliptical galaxy some 400 million light-years
from the earth. It belongs to a sub-class of galaxies that have an active
core (generally assumed to be a rotating super-massive black hole) with jets
of high speed particles apparently emanating from their poles. Like
Markarian 421, the first such object detected by the Whipple group as a
very-high-energy gamma-ray source, Markarian 501 is unusual in that the axis
of the jet happens to be pointing in our direction.

Because of the shielding effect of the earth's atmosphere, gamma rays must
generally be detected by Earth-orbiting gamma-ray telescopes such as
Compton. However, if the energy is sufficiently great they can be seen
indirectly with sensitive telescopes on mountain tops. Gamma rays are
photons with very high energy; the photons seen in these observations have
energies that are more than a million million times that of a photon of
visible light.

The very-high-energy gamma rays are produced in the interaction of cosmic
particles of even greater energy with ambient particles or photons in these
jets. The cosmic particles may be electrons or protons and they must be
accelerated by processes related to the enormous energy of the black hole to
energies in excess of those attained by man-made particle accelerators (the
so-called atom smashers such as those at Fermilab or Brookhaven). In these
cosmic particle accelerators the particle energies implied by the gamma-ray
energies are so great that theoretical astrophysicists are hard-pressed to
explain the processes involved. (If the primary particles are protons their
energies must be some millions times greater than the observed gamma rays!).
The problem is compounded by the extremely short duration of the flares seen
in Markarian 421 (one was less than 30 minutes in duration) which imply that
the emission regions are very small.

Even more difficult is to explain is how the gamma rays, once produced, can
escape from the environs of the jet without interacting with lower energy
photons and degrading in energy. Once clear of the galaxy the gamma rays
must traverse the vast regions of intergalactic space where they are
expected to be absorbed by interacting with infrared radiation from galaxies
formed in the early universe. However these observations suggest that the
density of infrared photons is less than was previously predicted.

The gamma rays are detected as distinct events as they strike the Earth's
upper atmosphere. Their collision with an air molecule generates a cascade
of light-emitting particles which can be detected by large optical detectors
such as the Whipple Observatory's 10-meter optical reflector.

The Whipple team, which involves scientists from the Smithsonian
Astrophysical Observatory, Iowa State University, and Purdue University, as
well as University College, Dublin, in Ireland and the University of Leeds
in the United Kingdom, pioneered the technique which identifies these gamma
rays from various background radiation. The same technique is used by the
Armenian-German-Spanish collaboration that operates the HEGRA array of
telescopes in the Canary Islands and by the French group that operates the
CAT telescope in the French Pyrenees.

The very-high-energy gamma-ray universe will be explored more fully by a new
ground-based telescope, VERITAS, proposed for construction in southern
Arizona and by the GLAST satellite, a proposed NASA mission, which would
extend the sensitivity of CGRO to higher energies.

A series of papers presented at the 4th Compton Science Symposium in
Willamsburg, Virginia, describes the recent activity in very-high-energy
gamma rays from these galaxies.



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From:	VBORMC::"[email protected]" "MAIL-11 Daemon" 27-MAY-1997 04:47:34.81
To:	[email protected]
CC:	
Subj:	[ASTRO] New gamma-ray count challenges astronomical theories

New gamma-ray count challenges astronomical theories

WEST LAFAYETTE, Ind. -- Scientists have discovered recently that there are
fewer low-energy photons in the universe than previously thought, an
observation that may alter the way astronomers think about how galaxies were
formed.

An international collaboration of astrophysicists has found that more
high-energy gamma rays are reaching Earth than expected from a distant
galaxy, Markarian 421, indicating that the gamma rays are not interacting
with and being absorbed by quite as many low-energy photons as they travel
through space.

"Using a new observational technique, we saw many more very-high-energy
gamma-ray photons from this source than we thought we would," says James
Gaidos, professor of astrophysics at Purdue University and a member of the
research team.

"We had believed there were more low-energy photons out there to absorb the
gamma rays, but so many are getting through to us from such a large distance
that it appears there's much less interaction taking place," he says.

John Finley, a member of the research team and assistant professor of
physics at Purdue, says, "Low-energy photons were created in the universe
during the time of galaxy formation, and the number of such photons we
observe, directly or indirectly, tells us how the galaxies formed.

"A reduced number of such photons has direct implications for current
theories of the history of the universe, and in particular for galaxy
formation," he says.

Gamma rays are highly energetic photons, and very-high-energy gamma rays
carry with them energies of trillions of electron volts, a level that is
hundreds of billions of times greater than visible light.

Gaidos says the latest observations present a mystery -- explaining how the
gamma rays, once produced, can escape from the environs of the galaxy
without interacting with lower-energy photons and degrading in energy. Once
clear of the galaxy, the gamma rays must traverse vast regions of
intergalactic space where they could be absorbed by interacting with
infrared radiation from galaxies formed in the early universe.

The results were presented today (Friday, 4/18) at the meeting of the
American Physical Society in Washington, D.C., by Jeff Zweerink of Iowa
State University for the Whipple Collaboration. The collaboration is a team
of scientists from the Smithsonian Astrophysical Observatory, Iowa State and
Purdue Universities, University College in Dublin, Ireland, and the
University of Leeds in the United Kingdom.

Markarian 421 is a giant elliptical galaxy 400 million light-years from
Earth. Scientists presume that the energy engine powering Markarian 421 is a
super-massive rotating black hole at the core of the galaxy, and they
theorize that the gamma rays may originate in very fast-moving jets of
matter that erupt from the region surrounding the black hole.

"Fortunately for us, the jet from this galaxy just happens to be pointed
right at Earth, which gives us a much better picture of the true numbers of
gamma rays associated with the jet," says Finley.

In the past four years, the team of astronomers working at the Smithsonian
Institution's Fred Lawrence Whipple Observatory in Arizona has observed
high-energy gamma-ray bursts coming from Markarian 421. The results
presented today are based on data from an unusually powerful flare of
very-high-energy gamma rays from Markarian 421 that was observed last May.

The Whipple research team pioneered a technique that differentiates these
gamma rays from background radiation originating from various sources in
space.

"Instead of using ground-based instruments to look at Markarian 421 when
it's directly overhead, we looked at the galaxy while it was on the
horizon," Gaidos explains. "From this position, the atmosphere is thicker
and absorbs more of the background radiation, which has much less energy
than these gamma rays.

"This technique allowed us to get a much cleaner signal and a more accurate
count of the number of very-high-energy gamma rays coming from this source.
It turned out to be much more than we expected," he says.

The gamma rays are detected as they strike the upper atmosphere. Their
collision with an air molecule generates a cascade of light-emitting
particles that can then be detected by large optical detectors, such as the
Whipple Observatory's 10-meter optical reflector.

NOTE TO JOURNALISTS: News releases announcing the Whipple Observatory
Gamma Ray Collaboration's discovery are being issued simultaneously by the
Harvard-Smithsonian Center for Astrophysics, Iowa State University and
Purdue University.



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From:	VBORMC::"[email protected]" "MAIL-11 Daemon" 27-MAY-1997 04:37:15.32
To:	[email protected]
CC:	
Subj:	[ASTRO] Evidence of a Nearby Supernova -- 4 Million Years Ago

Evidence of a Nearby Supernova -- 4 Million Years Ago

A team of astronomers led by Dr Martin Barstow of the University of
Leicester has used a group of white dwarf stars to probe the structure of
interstellar space in the vicinity of the Sun. Measurements made with the
Extreme Ultraviolet Explorer (EUVE) Satellite have revealed that the local
gas appears to be highly ionized in all directions. This can only have
happened as the result of a nearby supernova explosion and the relative
amounts of ionized gas indicate that it occurred about 4 million years ago.

The formation of the Solar System about 4500 million years ago, would have
been profoundly affected by the conditions existing in the cloud of dust and
gas from which it originated. Although this original nebula will have been
largely dissipated by intense radiation from the young Sun and has probably
been left behind by the motion of the Solar System through interstellar
space, more recent encounters with interstellar material may have affected
us directly and at the very least influenced our astronomical observations.

Our present picture of the local interstellar medium (the gas lying between
the stars out to distances of about 300 light years), is that the Sun is
embedded in and near the edge of a wispy diffuse cloud, known as the Local
Cloud (or Local Fluff). This cloud, which is only 20-30 light years across,
is itself in a larger much less dense region called the Local Bubble.
Interstellar gas near the Sun can only be studied directly by observing its
effect on the extreme ultraviolet (EUV) radiation from other stars. The
ROSAT EUV all- sky survey was able to map out the dimensions of the Local
Bubble but could not tell us anything about the state of the gas inside.

The current state of the gas in both the Local Cloud and the Local Bubble is
expected to bear the imprint of recent nearby events, such as supernova
explosions, and radiation from hot young stars. As a result the interstellar
gas should be ionized, with the electrons stripped from the constituent
(mainly hydrogen and helium) atoms. The ionized material can only be
detected in extreme ultraviolet spectra, recorded using NASA's Extreme
Ultraviolet Explorer (EUVE).

A critical question is whether the interstellar gas is in equilibrium, with
atoms being ionized at the same rate as the ions recombine with the
electrons, or not. At the observed level of ionization, the radiation from
nearby stars is not enough to maintain an equilibrium but the shortfall
could be made up of photons emitted by decaying dark matter.

A team of astronomers led by Dr Martin Barstow and including Paul Dobbie
(University of Leicester), Jay Holberg (University of Arizona), Ivan Hubeny
(Goddard Space Flight Centre) and Thierry Lanz (University of Utrecht), have
used the EUVE spectrometers to carry out detailed observations of 13 nearby
white dwarfs, using the shadowing effect of the interstellar medium on the
white dwarf spectra to measure the density and level of ionization.

Remarkably, while the gas density varies in different directions, the
fraction of material ionized is highly uniform. This can be best explained
by a non-equilibrium scenario in which the Local Cloud was ionized by the
shock wave from a nearby supernova explosion, since when the ions and
electrons have been slowly recombining. The observed fractions of ionized
hydrogen (27%) and helium (35%) indicate that the explosion occurred around
4 million years ago. This might be the same supernova which is believed to
have swept out the cavity we now identify with the Local Bubble. These
results confirm that a source of decaying dark matter is not needed to
explain the appearance of the local interstellar medium.

Dr Barstow is presenting these results at an International Astronomical
Union Colloquium on "The Local Bubble and Beyond", being held in Garching,
Germany, from April 21st to 25th inclusive. A paper on the topic was also
published in the 21 March issue of the Monthly Notices of the Royal
Astronomical Society.

Contact

     Dr Martin Barstow, Dept. of Physics & Astronomy, University of
     Leicester, Leicester LE1 7RH, UK.
     Phone: +44 (0)116 252 3492 (office), +44 (0)116 286 2330 (home);
     fax: +44 (0)116 252 3311; e-mail: [email protected]



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